Secondary recovery technique



Dec. 10, 1963 D. T. CAKES SECONDARY RECOVERY TECHNIQUE Filed Sept. 21.1960 INVENTOR. DAVID T. OAKES BY v United States Patent ChemicalCompany, St. Louis, Mo., a corporation of Delawwe Filed Sept. 21, 196i),Ser. No. 57,507 Claims. (Cl. 16)

The present invention relates to a method for increasing the recovery ofoil from subterranean petroleum bearing reservoirs. More particularly,the present invention relates to the method for recovering additionaloil from subterranean petroleum bearing reservoirs by means of secondaryrecovery techniques such as water flooding, in situ combustion, propaneinjection, etc.

During the primary production period of an oil field, oil is producedfrom the subterranean petroleum bearing reservoir as a result of thenatural pressures of the reservoir. These pressures may be sufi'icientto force oil from the reservoir all the Way to the top of the Well boreas in so-called flowing wells or they may be only sufiicient to displaceoil from the reservoir into the bottom of the well bore from where itmust be brought to the top by artificial means as pumping. Generally,the natural pressure will eventually decline to the extent that solittle oil is displaced from the reservoir that the operation of theproducing wells is no longer economically practical. At this point theprimary production period may be said to be ended and if additional oilis to be produced from the reservoir, resort must be made to theso-called secondary recovery techniques.

Secondary recovery techniques generally are those methods which supplyadditional energy to the reservoir for the purpose of moving oil to theproducing wells and, in general, tend to recreate the natural pressureor energy originally present in a subterranean petroleum bearingreservoir. These methods generally involve the injection of a gas orliquid into the reservoir and are exemplified by water flooding, COinjection, L.P.G. injection, in situ combustion, etc. Each of thesecondary recovery methods has specific advantages and types ofreservoir formations and crude oils to which they are particularlyadaptable. None of these methods, however, are at present totallyefficient in recovering all of the oil remaining after primaryproduction. This problem of incomplete recovery may be understood byconsidering one of the simplest secondary recovery procedures.

Water flooding secondary recovery is not completely effective inrecovering all of the oil which remains within the oil reservoir attermination of the primary production phase of operation. As water isforced into the oil bearing reservoir through an injection well, itspreads through out the formation in a gradually widening areadisplacing a bank of oil before it. Initially the water moves out in aroughly circular fashion. However, as the water-oil interface movesfurther from the injection well and nearer a producing well there is adecrease in the resistance to movement of the water-oil interface towardthe producing well. This results in a fingering or cusping of the waterfront toward the producing well and thereby causes a relatively earlybreakthrough of the water into the producing well. As a result of thisbreakthrough of Water into the producing well, there is an incompletesweeping of the oil reservoir by the water flood. After breakthrough,water is produced with the oil. Generally, when the water to oil ratioof the fluids produced from a producing well reaches 50:1 to 100:1 theWater flood is discontinued. At this point, which would generallysignify the termination of a secondary recovery water flood program,considerable oil still remains in the reservoir. The amount of this oilresidue has been estimated to be to percent and higher of the oilinitially present in the formation. It

ice

has been estimated, also, that water flooding and similar secondaryrecovery techniques are only effective in removing approximately 50percent of the oil which remained in the reservoir at termination of theprimary production period.

It is, therefore, an object of the present invention to provide a methodfor increasing the ultimate recovery of oil from oil bearing formations.It is a further object of this invention to provide a method forimproving the yields of oil recoverable from subterranean petroleumbearing formations by secondary recovery techniques. A specific objectof this invention is to provide a method whereby the yield of oilrecoverable by the Water flooding method of secondary recovery may beincreased. Additional objects will become apparent from the descriptionof the invention herein disclosed.

The aforementioned objectives are fulfilled in a manner which may bebest described by reference to the drawings which are a part of thisdescription.

Both FIGURES 1 and 2 present an illustrative plan view of a segment of amulti-well oil field. The numbers 1-13 and 21-33 inclusive indicatewells drilled into a petroleum bearing reservoir. It will be noted thatthese wells are arranged in a conventional S-spot arrangement of onewell in the center of a square with wells at each corner of the square.This technique of arranging wells is very conveniently employed insecondary recovery programs but is by no means a necessary arrangementto such recovery programs. FIGURE 1, which will be described first,represents the oil field as operated under conventional water floodingtechniques. FIGURE 2 will be used to describe the same oil fieldoperated under a water flood program developed in accordance with thepresent invention. In both figures the wells are, for the purposes ofthe present description, presumed to be no longer economicallyattractive to operate under the methods available during the primaryproduction period. Also for the sake of this description, it is to beassumed that. this reservoir is one which is particularly suited for theapplication of a water flood secondary recovery program.

Referring first to FIGURE 1, wells 4, 5, 9 and 10 are to be used aswater injection Wells. The water flood program is initiated by injectingwater into the reservoir through the injection wells, thereby forcingthe oil contained in the reservoir outward from the injection wellstoward producing wells 1, 2, 3, 6, 7, 8, 11, 12 and 13 through which theoil is withdrawn to the surface. After the water front has moved outwardto about midway between the producing wells and the injection well, thefront begins to finger or cusp toward the producing wells and at a latertime this fingering or cusping projects the water front into theproducing wells. Shortly after the entry of water into the producingwells, they can no longer be operated economically. This results in thediscontinuance of the water flood project. At this time the oil bearingreservoir will appear as in FIGURE 1. The unshaded portion representsthe area swept by the water flood and from which oil has been removed.The portion of the swept area designated A represents the fingering orcusping action of the water flood front. The shaded area B representsthe area unaffected by the water flood project. This portion of thereservoir is generally richer in oil than the original reservoir andcontains 30 to 40 percent of the oil originally present in thereservoir.

In describing the secondary recovery water flood as applied to the samefield in accordance with the present invention, reference will be madeto FGURE 2. Like FKGURE 1 the Wells 21, 22, 23, 26', 27, 28, 31, 32 and33 are producing wells and 24, 25, 29, and 36" are the initial injectionwells. Water is injected through the injection Wells and thereby oil isdriven through the reservoir toward the producing wells through which itis Li raised to the surface. This is continued until the wateroilinterface lies at a distance from the injection well represented by theline surrounding the unshaded swept area in FIGURE 2. This distance willvary for diifcrent reservoir formations and crude oils but may generallybe said to represent the distance to which the water-oil interface hasmoved when 10 to 90 percent of the water normally necessary to causebreakthrough of the water into the producing wells of a :given S-spotarrangement has been injected into the reservoir. When this point isreached, injection through the initial injection wells is ceased ormaterially reduced. in many cases it is desirable to continue to injectan amount of fluid sufiicient to prevent reverse movement of the initialflood front. After shutting-in the injection wells, wells number 21, 23,27, 31 and 33 are converted from producing wells to injection wells andwater passed therethrough into the reservoir. The new injection wellseach lie at one end of a potential unswept area if the field weresubjected to treatment as in FIGURE 1. By forcing water into theseinjection wells, the oil which would normally be left in the unsweptportions as in FIGURE 1, together with the additional oil which wouldhave been produced under conventional water "flood practices, is nowforced toward the remaining producing wells 22, 26, 28 and 32 asindicated by the arrows. By this method two advantages are gained.Irirst, the ultimate yield of oil from the reservoir is substantiallyincreased and second, considerably less water is needed per barrel ofoil to produce the oil through the more eificient application of theWater injected.

To further illustrate the present invention, the following specificexample is given. It is to be understood that this example is not in anymanner to be construed as limiting the objectives, conditions,operations or application of the present invention.

Two petroleum bearing formations are considered in this example, each 20acres in size and having a thickness of 20 feet with the same porosityand pore volume. Each contains wells drilled into them with the wellsbeing arranged in a conventional S-spot configuration as illustrated inFIGURES 1 and 2. Within each of the 20 acre formations is found 400,000barrels of displaceable oil or 1000 barrels per acre foot. In bothformations a Water flood secondary project was carried out until a 100:1ratio of water to oil is obtained in the producing wells. One of the 20acres formations is subjected to water floor in the following manner.Water is injected into the center well of the S-spot until there is abreakthrough of water into the producing wells. Injection is furthercontinued until the producing wells reach a water to oil ratio of 100to 1. The oil recovery is approximately 360,000 barrels and the amountof water utilized is 1,330,080 barrels.

The other 20 acre petroleum bearing formation is subjected to a waterfloor in accordance with the present invention. Water is injectedthrough the center well of the 5-spot until it is estimated that 50percent of the water necessary for normal breakthrough into theproducing wells has been injected. At this point, injection is stoppedthrough the injection well and it is shut-in and injection initiated intwo of the producing wells which are located opposite from one anotherin the S-spot arrange ment. Water injection is continued through thesenew injection wells until a 100:1 water to oil ratio is obtained in thetwo remaining producing wells. The amount of oil recovered in accordancewith this application of the present invention is 400,000 barrels. Thisrecovery represents a 100 percent contact and sweeping of the 20 acrearea by the injected water. Further, this represents an 11 percentincrease in oil recovery over the conventional water flood methodemployed in the other 20 acre S-spot. The amount of Water necessary forthe water flood project of this second 20 acre formation isapproximately 1,240,000 barrels or a 7 percent reduction in the amountof water.

The present invention may be summarized as a process for increasing theyield of petroleum through secondary recovery methods wherein water,gas, or other material is in'ected for the purpose of driving thepetroleum contained within the reservoir toward producing wells. Themethod of the present invention involves the injection of a liquid orgas through an injection well surrounded by a plurality of producingwells until some ti no prior to the breakthrough of the injected liquidor gas into the producing wells and then shuttingin the initialinjection well and thereafter rotating inection to alternate producingwells and Withdrawing oil from the remaining producing wells. The pointin time at which this rotation of injection takes place is expressed interms of percent of breakthrough. In most secondary recovery projectsthe amount of liquid or gas which will have been injected into thepetroleum bearing reservoir at breakthrough of the injected liquid orgas into the producing well is known or may be readily calculated. Thisamount of injection material when injected into the reservoir to thepoint at which breakthrough into the producing wells occurs wouldrepresent 100 percent of breakthrough. The point at which injection isbegun in the initial injection well would represent 0 percent ofbreakthrough. Therefore, rotation at 50 percent of breakthrough wouldmean a rotation of injection wells at a point in time at which 50percent of the amount of injection material necessary for breakthroughinto the producing wells has been injected. In practicing the presentinvention rotation should be carried out at 10 to 90 percent ofbreakthrough with a more efficient and preferred point of rotation beingbetween 20 to 80 percent of breakthrough, with a still more preferredpoint of rotation being from 40 to percent of breakthrough. It will beunderstood, of course, that for different reservoir formations theoptimum point of rotation expressed as percent of breakthrough willvary.

Since the operability of the present invention is based on the rotationof injection wells at some time relative to the amount of materialinjected, the injection rates will generally be of no particularimportance. Injection may be at any rate suitable for the givenformation and the gas or fluid being injected.

Throughout the preceding description of the present invention, referencehas been made primarily to the 5-spot arrangement of wells. This was forthe purpose of simplification of description and should not be construedas limiting the present invention. The novel secondary recoverytechnique of this invention may be readily adapted to a 7-sp-ot pattern,a 9-spot pattern, a line drive, a staggered line drive, or to any otherpattern or arrangement of wells.

When the initial injection wells are shut-in, it will often be necessaryto maintain injection of a very small amount of the injection materialin this well. The purpose of this is to maintain sufiicient positiveforce on the original oil-injection material interface that there willbe no reversal in direction of this interface due to injection in thenew injection wells. The amount of material injected during this periodis insignificant as compared to the initial injection rate. This rate ofinjection will vary considerably with the physical characteristics ofthe oil bearing formations, injection rates of the new injection wells,and type of secondary recovery method being used, and is thereforeobviously not critical. Those skilled in the art will have nodifi'iculty in calculating these injection rates.

Though the present invention finds its greatest applicability in beingutilized in water flood secondary recovery programs, it is certainly notlimited thereto. It may be equally well utilized with any secondaryrecovery method wherein liquid or gas injection is necessary. Such otherliquids and gases are liquified petroleum gases, propane, air for insitu combustion, carbon dioxide, carbonated water, etc. In addition toliquids and/ or gases, any material classed as a fluid may be utilized.Fluid in its popular sense is considered as referring to liquids andmore generally as anything that has fluidity. Herein it is meant toencompass not only these more conventional meanings but also refers tothat physical state of a system which may be defined either as a liquidor gas or neither, this frequently occurring where the pressure and/ ortemperature is greater than the critical pressure and/ or temperature ofthe system. In the event the present invention is used in othersecondary recovery methods, certain modifications may become necessarybut as such are within the ability of those skilled in the art andhaving the present disclosure as reference. They may be made andutilized without departing from the spirit and scope of the presentinvention. The optimum rotation point will vary somewhat as thesecondary recovery method varies. However, the ranges of operabilitywill remain within the ranges of rotation points given previously inthis specification.

What is claimed is:

1. A method of recovering oil from a multi-well oil bearing reservoirwherein a fluid is injected into the reservoir through a plurality ofinput wells, each input well being disposed centrally relative to aplurality of adjacent producing wells, and after injection has begunshutting in the injection wells at some time prior to the breakthroughof the injection fluid into the adjacent producing wells and thenrotating injection to alternate adjacent producing wells and thereafterwithdrawing oil from the remaining adjacent producing wells.

2. A method of recovering oil from a multi-well oil bearing reservoirwhere-in a fluid is injected into the reservoir through a plurality ofinput wells, each input Well being disposed centrally relative to aplurality of adjacent producing wells, continuing the injection of thefluid until to 90 percent of the volume of the fluid necessary to causebreakthrough of the fluid into the adjacent producing wells has beeninjected and then rotating injection to alternate adjacent producingwells and thereafter withdrawing oil from the remaining adjacentproducing wells.

3. A method of recovering oil from a multi-well oil bearing reservoirwherein a liquid is injected into the reservoir through a plurality ofinput wells, each input well being disposed centrally relative to aplurality of adjacent producing wells, continuing the injection of theliquid until 10 to 90 percent of the volume of the liquid necessary tocause breakthrough of the liquid into the 5 adjacent producing wells hasbeen injected and then rotating injection to alternate adjacentproducing wells and thereafter withdrawing oil from the remainingadjacent producing wells.

4. The process of claim 3 wherein the liquid is water.

5. The process of claim 3 wherein the liquid is selected from the groupconsisting of liquefied petroleum gases, liquefied carbon dioxide, andcarbonated water.

6. The process of claim 3 wherein injection is rotated at a time when 20to percent of the liquid normally necessary to cause breakthrough ofliquid into the pro ducing wells has been injected.

7. A method of recovering oil from a multi-well oil bearing reservoirwherein a gas is injected into the reservoir through a plurality ofinput wells, each input well being disposed centrally relative to aplurality of adjacent producing wells, continuing the injection of thegas until 10 to percent of the volume of the gas necessary to causebreakthrough of the gas into the adjacent producing wells has beeninjected and then rotating injection to alternate adjacent producingwells and thereafter withdrawing oil from the remaining adjacentproducing wells.

8. The process of claim 7 wherein the injection gas is one selected fromthe group consisting of carbon dioxide, air, and natural petroleumgases.

9. The process of claim 7 wherein injection is rotated at a time when 20to 80 percent of the gas normally necessary to cause breakthrough of gasinto the produc ing wells has been injected.

10. In a secondary recovery method of recovering oil from a multi-welloil bearing reservoir in which an injection fluid is injected into aplurality of injection wells, each injection well being surrounded by aplurality of producing wells adjacent thereto from which oil iswithdrawn, an improvement comprising shutting-in the original injectionwells to an injection rate just sufficient to maintain the minimumpressure necessary to prevent reversal of the injection fluid-oilinterface, the original injection wells being shut-in at a time whenfrom =10 to 90 percent of the fluid normally necessary to cause abreakthrough of fluid into the producing wells has been injected intothe reservoir and subsequently rotating injection to alternate adjacentproducing wells and thereafter withdrawing oil from the remainingproducing wells.

References Cited in the file of this patent UNITED STATES PATENTS2,347,778 Heath May 2, 1944 2,798,556 Binder et a1 July 9, 19572,885,002 Jenks May 5, 1959 2,885,003 Lind-auer May 5, 1959

1. A METHOD OF RECOVERING OIL FROMA MULTI-WELL OIL BEARING RESERVOIRWHEREIN A FLUID IS INJECTED INTO THE RESERVOIR THROUGH A PLURALITY OFINPUT WELLS, EACH INPUT WELL BEING DISPOSED CENTRALLY RELATIVE TOAPLURALITY OF AD-